Much work has been done directed to the goal of finding a suitable material to repair or replace tissue defects. Regarding the skeletal system, Coetze (1980) reported upon the regeneration of bone in the presence of calcium sulfate. Coetze reported that although autogenous bone would be recognized as the ideal grafting material, it is not always available and it cannot be molded in various shapes and sizes. With calcium sulfate, a large mastoid cavity can be eliminated and a new antrium created. Calcium sulfate is available in all countries, is easy to handle, is inexpensive, and above all, is replaced with autogenous bone as nature designed.
The use of bioresorbable bone void filler devices such as calcium sulfate pellets have been reported in the literature for many years. The pellet dimensions vary in size from 4 to 7 mm in diameter and from 3 to 6 mm in thickness. The use of these pellets resulted in satisfactory bone void filling, the pellets resorbing as new bone grew. Pellets were preferred over a solid or compact mass because it was perceived that they would resorb faster, facilitate bone and void filling, and offer more surface area for drug elution. Also, a solid mass would resorb from the outside to the inside, a slow process. However, the main problem remaining with infected or destroyed bone is to replace the bone. The goal is to restore normal bone and replace the normal features. Thus, controlled regeneration of bone is desired.
Several patents have been issued towards achieving the goals mentioned above. For example, U.S. Pat. No. 4,619,655 to Hanker et al., issued Oct. 28, 1986, discloses the use of Plaster of Paris to form implants as well as a bioresorbable scaffold for implants and bone repair in animals. The plaster is mixed with a non-bioresorbable calcium source such as calcium phosphate ceramic to stimulate bone formation. Plaster is also proposed to be used as a medicament implant, encasing the active material for subsequent release in situ.
U.S. Pat. No. 5,147,403 to Gitelis, issued Sep. 15, 1992, discloses a technique for implanting a prosthesis in a host bone by preparing the surface of the host bone to receive the prosthesis, applying calcium sulfate in free-flowing form to the receiving surface of the host bone, and seating the prosthesis in the receiving surface whereby the calcium sulfate fills one or more gaps resulting between the prosthesis and host bone.
U.S. Pat. No. 5,569,308 to Sottosanti, issued Oct. 29, 1996, discloses methods for use in bone tissue regeneration containing a barrier material and a graft material. The barrier material can be calcium sulfate, the graft material being any suitable material, including a composite graft material containing demineralized, freeze dried, allogenic bone, and calcium sulfate.
U.S. Pat. No. 6,614,206 to Randolph et al., issued Mar. 25, 1997, discloses the controlled release of calcium sulfate, as well as the controlled release of an additive to calcium sulfate matrix such as medicaments or pesticides. The controlled release is achieved by a pellet comprising calcium sulfate.
In view of the above, the direction of research and development has gone towards the use of pellets, made from calcium sulfate and possibly containing medicaments or other additives, to provide a bioresorbable scaffolding for implants and to form the implants themselves.
Due to its poor mechanical properties, Plaster of Paris (POP or calcium sulfate) was almost discarded from the biomedical field for the past 30 years. Research was focused on new materials such as HA and collagen (both non-bioresorbable materials), bioresorbable polymers (generally releasing irritant by-products) and the like. These osteoconductive materials were shaped to imitate the porosity of the cancellous bone structure. Porous material structure includes thin partitions of compact material separating empty cells which have an average dimension of more than 100 micrometers. Bone cells were expected to migrate through the porous structure and integrate the implant. Generally, only the periphery of the implant hosts living cells. The core stays untouched if too deep. When these materials are used in a solid or compact form, they are proposed as small pellets or particulate. Solid or compact material is defined as non-porous or micro-porous material; with average pore size less than 20 micrometers. In the case of biodegradable polymers, such as PLA-PGA polymers, a porous structure is also preferred because large quantity of material can become toxic during resorbsion. Sometime, holes were drilled through these porous implants as attachment sites or to facilitate bone cell migration. All large implants observed by applicant were made of porous materials.
Small pellets are hard to handle, implant, locate, and shape into the tissue cavity. Once implanted, they can migrate from the implantation site. Larger pellets are too big to fit small details of the cavity features. They also dissolve slower from outward to inward.
There is a renewed interest in POP. First, it resorbs at a rate comparable to new bone growth and the final result is a cavity filled with actual patient bone instead of a composite new bone-bone void filler material. Second, POP is the only commonly known material which can be implanted in an infected wound (any other material will generate a foreign body reaction). Third, POP is a pure, clean mineral that does not present the imnmunoreaction potential other implants can cause.
It would be advantageous to derive a scaffolding implant having preformed structure but still possessing architectural similarities to the prior art pellet implants discussed above.